Chapter 3: The Biosphere
Warm/cold/dry/wet variations on earth – organism patterns because of that variation.
What the world is made up of biologically
Overview of the diff types of systems in the world and how they relate to the climatic
factors in ch.2
The biosphere is the zone of life on Earth – found everywhere on earth.
Biomes are large-scale biological communities shaped by the physical environment, particularly
Biomes are categorized by dominant plant forms in that region - not taxonomic/relatedness
o Tundra, boreal forest, grassland, ect.
Dominant plants are visible, immobile, and occupy sites for a long time
o Thus, they are good indicators of the physical environment, reflecting climatic conditions
and disturbances that have occurred in that biome.
Because it can’t move, it must be able to tolerate all the conditions of that site
during all times of the year
Terrestrial biomes are characterized by growth forms of the dominant plants, such as leaf
deciduousness (shedding leaves) or succulence (ability to hold water - catcus)
Plant growth forms
Deciduous trees: loose their leaves during the cold/dry
-south western ontario
Needle-leaved evergreen trees: retain their
photosynthetic tissues year-round
- more as you move north Cacti & Shrubs: succulent steams and leaves contain
water storage tissues
Grasses & Sedges: nonwoody & grow from the base of
Has good implications for grazing or fires, because it
doesn’t affect the grass, just removes the leaves on top.
Evergreen broad-leaved trees: in tropical regions
carry out photosynthesis year round
But do not loose their leaves like deciduous trees
Scierophyllous shrubs: have tough, leathery leaves
Forbs: board-leaved herbaceous (nonwoody & nongrass)
Ex. Dandelions Plants have taken many forms in response to selection pressures: such as aridity(dryness),
extreme temperatures, intense solar radiation, grazing, and crowding.
Similar growth forms can be found on different continents, even though the plants are not
genetically related. – similar adaptations will be in the same biome.
o Convergence: Evolution of similar growth forms among distantly related species in
response to similar selection pressures.
Temperature has direct physiological effects on plant growth form.
o The interaction of precipitation and temperature act together to influence water
availability and water loss by plants.
Water availability and soil temperature determine the supply of nutrients in the soil.
Biomes Vary with Mean Annual Temperature and Precipitation
Problem: this does not account for seasonal variation which can have large effects (ex. due to
tilt of the earth) Global Biome Distributions *memorize
Global Biome Distributions Are Affected by Human Activities
Human activities influence the distribution of biomes.
o Land use change: Conversion of land to agriculture, logging, resource extraction, urban
The potential and actual distributions of biomes are markedly very different – we’ve
modified/effected many biomes
A lot of the earth is ~40% changed
from what it once was
The parts of the world that haven’t
been modified are the least desirable
places – boreal, tundra, desserts,
There are nine major terrestrial biomes.
Climate diagrams show the characteristic seasonal patterns of temperature and precipitation at
a representative location. Tropical Rainforests
WET & HOT - humid
Mean temperatures every month is
Stable high precipitation rates year round
~10% of earths surface & contain ~50%
of Earth’s species.
-- High biomass, high diversity—
Light is a key factor—plants must grow very tall above their neighbors or adjust to low light
levels. – because its so humid, there’s a bunch of different mechanisms available:
o Emergents rise above the canopy: adapt to grow very tall
o Lianas (woody vines) and epiphytes use the other trees for support.
Normally growing on a tree would be a hostile environment in other systems, but
its so moist that it’s not hindering here.
o Understory trees grow in the shade of the canopy: adapt to low light conditions
o shrubs and forbs occupy the forest floor.
This area is productive enough that there are many different layers of plants
Tropical rainforests are disappearing due to logging and conversion to pasture and croplands.
o ~½ of the tropical rainforest biome has been altered.
Recovery of rainforests is uncertain: Soils are nutrient-poor, and recovery of nutrient supplies
may take a very long time.
o The soill is very old – never went through the last glaciation, so the soil is sucked dry of
all its nutrients
o So when you get rid of all the above ground plants – burn/clear them, it’ll take a long
time to recover from that
o Example of this: Fish-bone pattern caused by logging roads – can see an extensive
clearing of rainforest over a short time period. Tropical Seasonal Forests and
Located slightly N/S of equator
o Wet and dry seasons associated
with movement of the ITCZ.
Yellow on graph = drought/water stress
Still stable warm temperature
Different plants than in the rain forest:
o Shorter trees
o Strategies they use to deal with the dry seasons = deciduous - drop leaves in dry
o See other strategies being more competitive: more grasses and shrubs.
Tropical seasonal forests = more trees – more spares
Tropical season savannas = more grasses, fewer trees
How to convert a forest to a savanna:
Mostly due to climate factors, but also:
o Fires promote establishment of savannas; some are set by humans.
Because grasses are more favorable/tolerant to fire and can grow back quicker
o In Africa, large herbivores—wildebeests, zebras, elephants, and antelopes—also
influence the balance of grass and trees – (by eating them)
o On the Orinoco River floodplain, seasonal flooding promotes savannas.
Less than 1/2 of seasonal tropical forests and savannas remain.
o Human population growth in this biome has had a major influence.
o Large tracts have been converted to cropland and pasture. Hot Deserts:
High temperatures, low moisture.
Location: at high pressure systems –
areas of subsidence
Yellow = drought
o Low water availability constrains
plant abundance and influences
o Sparse vegetation and animal
o Cannot support a high biomass
When it DOES occasionally rain, plants take
advantage of this by growing, flowering, &
seeding very quickly while it still remains moist
Convergence in the Forms of Desert Plants
Many plants have succulent stems that
There are such strong selection pressures in
desserts that you see a lot of convergence
Convergence of this form is shown by
cacti (Western Hemisphere) and euphorbs
- they look & act similar, but they are
very distantly related. Humans use deserts for agriculture and livestock grazing. – try to make them more productive
o Agriculture depends on irrigation, and results in soil salinization.
Dessert soil have high [salt] b/c high evaporation and plants can’t handle that
o Long-term droughts and unsustainable grazing can result in desertification—loss of all
plant cover and soil erosion
Problem: in Africa the population depends on that agriculture – get famine
Great plains in Europe & North America
Warm, moist summers and cold, dry
o Blue graph = cold temp force
adaption’s on these plants
Grasses dominate; maintained by
frequent fires and large herbivores such
as bison (historically)
o Stop fires = more forests
- Grasses grow from the base
- to cope with dry conditions, more roots than stems
o the deep root systems results in accumulation
of organic matter and high soil fertility.
- Most fertile grasslands of central North America and
Eurasia have been converted to agriculture b/c rich soil
In arid grasslands, grazing by domesticated animals can exceed capacity for regrowth, leading
to grassland degradation and desertification.
o Irrigation in some areas causes salinization. Temperate Shrublands & Woodlands
Mediterranean life – Greece, Italy, Californi